This invention relates to Raman amplification, for example in optical amplifiers or fiber lasers, and particularly to optical amplifiers for amplification across a wide wavelength band.
Various rare-earth doped optical amplifiers are known, such as Erbium or Erbium-Ytterbium doped fibers, and these are used to compensate for the fiber link and splitting losses within optical communications systems. Pump light from a pump source is used to excite the dopant atoms in the fiber. Emission of energy from the excited atoms is stimulated by the incident signal, and this emission results in amplification of the signal.
The gain of rare-earth doped fibers as a function of the wavelength of the signal to be amplified typically includes a maximum gain in the form of a plateau, which provides the most useful operating region of the amplifier. It is desirable to provide a flat gain over the operating wavelength range, and various gain flattening filters are used for this purpose. However, the width of the plateau in the gain profile provides a limitation to the range of wavelengths for which the amplifier can be used.
The characteristics of practical amplifiers have lead to the definition of three wavelength bands: the S band (1450 nm-1520 nm); the C band (1527 nm-1563 nm); and the L band (1570 nm-1603 nm). A 7 nm guard band is provided between the bands. Different rare-earth dopants will provide different gain profiles, and amplifier arrangements have been proposed which place different types of amplifier in parallel, so that amplification across a broader wavelength range can be achieved. However, practical rare-earth amplifiers have not yet been developed for the S-band.
A Raman amplifier is another known amplifier configuration. This amplifier uses conventional fiber, which is may be co- or counter-pumped to provide amplification over a wave range which is a function of the pump wavelength. The Raman amplifier relies upon forward or backward stimulated Raman scattering. Typically, the pump source is selected to have a wavelength of around 100 nm below the wavelength over which amplification is required. This type of amplifier has the advantage that it does not attenuate signal outside the wavelength range over which amplification takes place, and can also be used amplifying a wide range of wavelengths, including the S-band.
A first problem with the Raman amplifier is the need for the pump source to be provided at around 100 nm below the amplification band. When a Raman amplifier is used for amplifying the S-band, the required pump wavelength will fall close to the water peak of the fiber (typically between 1375 nm and 1385 nm). As a result, there is a high level of attenuation of the pump signal, so that a high power pump is required, for example approximately 3 W.
Although fibers are available with water peaks removed, these are not suitable for S-band Raman amplifiers because the dispersion zero wavelength falls within the band of amplification. Within fiber lasers, water peak generation can be reduced by making gratings using Deuterium as a sensitising agent instead of Hydrogen. This can be prohibitively expensive. There is therefore a need to reduce the effect of the water peak for a Raman amplifier being used for the S-band and for Raman fiber lasers.
A second problem with Raman amplifiers is the need for high power pump sources. In particular, in order to obtain a flat gain profile over the wavelength range of interest (which may be any of the wavelength bands), multiple pump wavelengths are required, and with individually selectable pump powers. This has in the past required a number of high power pump sources, and it may be difficult in practice to implement pump sources of the required pump wavelength and power.
According to a first aspect of the invention, there is provided a Raman optical amplifier comprising a fiber and a pump source arrangement for providing first and second pump source signals to the fiber, the first pump source signal having a wavelength between 1355 nm and 1375 nm and the second pump source signal having a wavelength between 1385 nm and 1405 nm.
This arrangement provides two pump wavelengths, one on either side of the water peak wavelength of the fiber. This enables lower power pump sources to be used to for S-band amplification, and thereby improves the gain and efficiency of the amplifier. This avoids the need to use specialised fibers with the water peak suppressed.
The first pump source signal may have a wavelength of approximately 1365 nm and the second pump source signal may have a wavelength of approximately 1389 nm. These are thus on either side of the typical water peak region of 1375 nm to 1385 nm.
The first pimp source signal may have a power of 300 mW-800 mW and the second pump source signal may have a power of 300 mW-600 mW.
According to a second aspect of the invention, there is provided a Raman fiber laser comprising a fiber forming a laser cavity, a pump source, and a plurality of gratings at one or both ends of the laser cavity for reflecting selected wavelengths, wherein one of the gratings is arranged to reflect a signal wavelength below 1375 nm and the next grating in the path of a signal within the cavity is arranged to reflect a wavelength above 1385 nm.
In this way, the wavelength shifts within the laser cavity avoid the water peak band of wavelength, thereby reducing the high attenuation within that wavelength band.
The invention also provides a method of providing Raman optical amplification, the method comprising:
providing first and second pump source signals to a fiber, the first pump source signal having a wavelength between 1355 nm and 1375 nm and the second pump source signal having a wavelength between 1385 nm and 1405 nm.
According to a third aspect of the invention, there is provided a Raman optical amplifier comprising a fiber, a first pump source arrangement for providing a pump source signal to the fiber and a second pump source arrangement for providing a plurality of pump source signals. The first pump source provides a first pump source signal with a first wavelength and a first power, and the signals of the second pump source arrangement each have a power lower than the first power and a wavelength in a range approximately one Stokes shift higher than the first wavelength. The second pump source signals are thereby amplified by the Raman effect by the first pump source signal, and these amplified second pump source signals in turn cause signal amplification.
This arrangement provides a single high power pump source, which results in amplification of a second group of lower power pump signals. These have the desired wavelengths and intensities so that after they have been amplified, they provide the required signal amplification. The first power is preferably greater than 2 W, and the power of each pump source signal of the second pump source arrangement is less than 10 mW. The second pump source arrangement can then comprise a plurality of semiconductor lasers, whereas the high power first pump will comprise a fiber laser.
The first wavelength may be approximately 1380 nm, and the second wavelengths may be in the range 1420 nm to 1500 mm. This provides a scheme suitable for amplifying the C-band, although other wavelengths may be selected to enable amplification of the other bands.
The third aspect of the invention also provides a method of providing Raman amplification comprising:
providing first a pump source signal in a first wavelength range; and
providing second pump source signals in a wavelength range approximately one Stokes shift higher in wavelength, the second pump source signals being amplified by the Raman effect as a result of the first pump source signal,
wherein a signal is amplified by the Raman effect as a result of the amplified second pump source signals.
The amplifiers of the invention can be used in a wavelength division multiplex (WDM) optical communications system comprising a transmitter for generating signal radiation of wavelength in an operating wavelength range, a receiver for receiving for detecting the signal radiation, and an optical fiber link between the transmitter and the receiver. One or more of the optical amplifiers are provided in the link.